1. Field of the Invention
The present invention relates to a multiple-format video encoder that can encode RGB signals in various video formats such as the NTSC (National Television System Committee) format and the PAL (phase-alternation line) format.
2. Description of the Prior Art
A device called a multiple-format video encoder is used to generate a color-difference signal from RGB signals and then generate a carrier chrominance signal and other signals by modulating a chrominance subcarrier with the color-difference signal in a video format that conforms to the NTSC, PAL, or other system. For example, an encoder designed for the NTSC system generates from RGB signals fed thereto a luminance signal Y and a carrier chrominance signal C in accordance with the formulae (1) and (2) below.Y=0.299 R+0.587 G+0.114 B  (1)C=[(B−Y)/2.08]·sin ωt+[(R−Y)/1.14]·cos ωt  (2)where ω equals to 2πfsc, where fsc represents the frequency of the chrominance subcarrier, which, for the NTSC system, equals to 3.579545 MHz.
In a digital encoder that generates from digital RGB signals fed thereto a digital luminance signal Y and a digital carrier chrominance signal C, for example in a case where the digital encoder receives the digital RGB signals at 13.5 MHz, the values of sin ωt and cos ωt at various phases at 13.5 MHz are stored in a ROM (read-only memory), so that, when the carrier chrominance signal C is generated in accordance with the above-noted formulae, the values of sin ωt and cos ωt are read out sequentially from the ROM in synchronism with 13.5 MHz clock pulses.
Specifically, as shown in FIG. 1, the interval between two adjacent pulses in the 13.5 MHz clock pulses of the encoder corresponds to a phase difference Ph of 360×3.579545/13.5=95.45 (degrees) in the chrominance subcarrier. Accordingly, the values of sin ωt at phases S0, S1, . . . are stored in the above-mentioned ROM. Similarly, the values of cos ωt, whose curve is different only in phase from the curve of sin ωt shown in FIG. 1, are determined and stored in the above-mentioned ROM. The stored values are read out sequentially, i.e. in order of S0, S1, . . . , in synchronism with the clock pulses to generate the chrominance subcarrier.
The ratio fsc/13.5 MHz of the frequency of the chrominance subcarrier to the frequency 13.5 MHz of the clock pulses equals to 35/132. Accordingly, in the above-mentioned ROM, a total of 132 values of the sine function, i.e. corresponding to phases S0, S1, . . . , S131, are stored, and the values are read out sequentially and recurrently, i.e. from S0 to S131, and then again from S0 to S131, and so forth. Thus, in the above-mentioned ROM are stored the values that correspond to 35 cycles of the chrominance subcarrier.
The above-described relationship as actually observed in the NTSC system is shown in the column headed “NTSC” in Table 1 below. In Table 1, the “number of words” represents the number of values, for each of the sine and cosine functions, that are stored in the above-mentioned ROM; that is, one value of the sine function or one value of the cosine function is stored per one word.
TABLE 1NTSCPALPAL-MPAL-Nfsc3.5795454.433618753.575611493.58205625(MHz)fsc/≈35/132≈423/1288≈303/1144≈173/65213.5MHzPhase95.45118.2395.3595.52Diff-erence(°) perClockPulseIntervalNumber13212881144652ofWords
Different video formats are used in different regions of the world, such as the NTSC, PAL, PAL-M, PAL-N, and other systems. Different video systems use different frequencies for the chrominance subcarrier from which the carrier chrominance signal is generated. For this reason, in a conventional multiple-format video encoder that is composed of digital circuits and that is designed to cope with a plurality of video formats, it is inevitable to store different sets of values of the sine and cosine functions separately for different video formats.
For example, in the PAL system, as shown in the column headed “PAL” in Table 1, the frequency fsc of the chrominance subcarrier is 4.43361875 MHz, and therefore, in a cases where the encoder receives RGB signals at 13.5 MHz, the frequency ratio fsc/13.5 MHz approximately equals 423/1288. Accordingly, one clock pulse interval corresponds to a phase difference of 360×423/1288=118.23 (degrees), and therefore the ROM needs to be capable of storing a total of 1288 words for each of the sine and cosine functions.
In the PAL-M system, the frequency fsc of the chrominance subcarrier is 3.57561149 MHz, and therefore, in a cases where the encoder receives RGB signals at 13.5 MHz, the frequency ratio fsc/13.5 MHz approximately equals 303/1144. Accordingly, one clock pulse interval corresponds to a phase difference of 95.35 (degrees), and therefore the ROM needs to be capable of storing a total of 1144 words.
In the PAL-N system, the frequency fsc of the chrominance subcarrier is 3.58205625 MHz, and therefore, in a cases where the encoder receives RGB signals at 13.5 MHz, the frequency ratio fsc/13.5 MHz approximately equals 173/652. Accordingly, one clock pulse interval corresponds to a phase difference of 95.52 (degrees), and therefore the ROM needs to be capable of storing a total of 652 words.
Thus, in a conventional multiple-format video encoder, to cope with the NTSC, PAL, PAL-M, and PAL-N formats, which use different frequencies fsc for the chrominance subcarrier, it is necessary to provide separate ROMs for different formats, and store in those ROMs a total of 132+1288+1144+652=3216 words for each of the sine and cosine functions. Consequently, a conventional multiple-format video encoder suffers from comparatively high costs resulting from the large capacity required in the ROMs.
It is also customary to group the addresses available on a single ROM into a plurality of areas so that values of trigonometric functions for different video formats used for encoding are stored in different areas on a single ROM. Even in this case, the areas for different video formats are not in any way related with each other, and therefore this method should be regarded as equivalent to providing separate ROMs.